1. Field of the Invention
The present invention relates to a timepiece bearing, a movement, and a portable timepiece.
2. Description of the Related Art
Conventionally, a rotating mechanical component such as a gear used in a portable timepiece, such as a wristwatch or a pocket watch, is formed such that a bearing is arranged so as to contain rotation shaft ends thereof and that the rotation shaft is rotated while guided by the bearing to transmit torque, thereby ticking away the time.
Here, as the construction of a conventional timepiece bearing, a construction as shown in FIG. 21 is known (See, for example, JP-A-2004-294320). FIG. 21 is a sectional view of a balance with hairspring.
As shown in FIG. 21, a balance with hairspring 520 includes a balance staff 523 whose thin small-diameter portions 521 and 522 at both ends thereof are supported so as to be rotatable around a center axis C by a timepiece bearing 510 formed in a balance bridge 505 and a main plate 504 so as to extend along the center axis C, a balance wheel 528 equipped with an annular rim portion 524 constituting a balance wheel main body and an arm portion 525 whose both ends are connected to the rim portion 524 and which extends in the diametrical direction of the rim portion 524, with an intermediate portion 526 of the arm portion 525 being fixed to a central shaft portion 527 of the balance staff 523, a collet 550, and a double roller 554 retaining an impulse pin 552.
The timepiece bearing 510 has an outer side bearing frame 512 retained by the inner peripheral surface of the balance bridge 505, an inner bearing frame 511 arranged inside the outer bearing frame 512, a hole jewel 514 arranged in a medium diameter recess of the inner bearing frame 511 and serving as a journal bearing for the small diameter shaft portion 522 at the upper end of the balance staff 523, a cap jewel 515 arranged in a large diameter recess of the inner side bearing frame 515 and serving as a thrust bearing for the small diameter shaft portion 522 of the balance staff 523, and a presser spring 516 locked to a groove of the outer side bearing frame 512 and retaining the cap jewel 515 in the large diameter recess of the inner side bearing frame 511.
To permit rotation of the shaft, the above-described conventional timepiece bearing 510 requires a gap or space between the shaft (the small diameter shaft portion 522) and the bearing (the cap jewel 515). Due to the presence of this space, when the attitude of the timepiece is changed or an impact is applied thereto, the position of the shaft fluctuates. Then, the torque transmitted from a barrel drum to the balance with hairspring fluctuates, resulting in fluctuation in the oscillation angle and the rate. As a result, the time indication accuracy of the timepiece deteriorates.
The present invention has been made in view of the above problem. It is an object of the present invention to provide a timepiece bearing, a movement, and a portable timepiece helping to achieve an improvement in terms of time indication accuracy.